Automatic balancing devices for counterbalancing out-of-balance masses are known in many different applications. However, the most complex out-of-balance situations occur when both the position and size of the out-of-balance mass is unpredictable and the speed of rotation is variable, as in the case of a washing machine. Many different automatic balancing devices have been proposed and used in washing machines and many of these are effective at counterbalancing out-of-balance masses at speeds above the critical speed (ie. the speed of resonance of the system). Examples of this type of automatic balancing device are shown in GB1,035,033; GB1,092,188; WO 93/23687; WO 95/32372; U.S. Pat. Nos. 5,813,253; 5,862,553; DE1 912 481. All of the devices shown in these documents make use of the phenomenon by means of which, at speeds of rotation above the critical speed, freely rotatable counterbalancing masses automatically take up positions in which the out-of-balance mass is counterbalanced. However, it is recognised in some of the aforementioned documents that, at speeds below the critical speed, freely rotating counterbalancing masses act so as to exacerbate the excursion of the rotating body due to the presence of the out-of-balance mass. In these cases, the counterbalancing masses are proposed to be locked in a fixed position with respect to the chamber in which they are located when the body is rotating at a speed below the critical speed. In the case of U.S. Pat. No. 5,813,253, a roller locates in a recess in order to prevent the balancing masses from moving along the annular path in which they run. The roller is released from the recess when the body exceeds the critical speed. In GB1,092,188, the counterbalancing masses are pivotably mounted about an axle with locking members provided to lock the masses in a fixed position with respect to the chamber in which they are housed when the speed of rotation is below critical. The locking means release when the speed is above critical. It is also envisaged in this prior art document that the counterbalancing members could be locked together, so that they have a zero net out-of-balance effect until the critical speed has been exceeded.
The known mechanisms by means of which counterbalancing masses can be locked in a position in which they have a zero net out-of-balance effect are generally difficult and expensive to manufacture. They are susceptible to damage in view of the movement of the counterbalancing masses, which can sometimes be quite violent. However, in arrangements which do not lock the counterbalancing masses, acceleration of the drum of the washing machine from a below-critical speed to an above-critical speed can cause extreme excursion, especially at the critical speed.